1. Field of the Invention
The present invention generally relates to semiconductor junction devices and, in particular, to junctions which exhibit quantum mechanical tunnelling.
2. Description of the Prior Art
It has long been understood and accepted that the behavior of electrons generally can be partially described as discrete particles which carry a discrete charge. Other behaviors of electrons can be more aptly described and explained as waves or wave packets. It has been similarly understood that most expressions of fundamental behavior of electrons in electrical currents are descriptions of gross behavior of large numbers of electrons and neglect the effects of behavior of individual electrons, which, when described, are usually expressed statistically. Numerous devices which exploit the statistically expressed quantum mechanical behavior of electrons have been developed, such as the Esaki, or tunnel, diode.
Tunnel diodes have an extremely narrow depletion region, forming a barrier to electron movement. However, under conditions of very low forward bias, a statistically predictable proportion of conduction electrons can initially tunnel through the junction from an energy level in the conduction band to an equal but empty energy level in the valence band, producing a differential negative resistance effect as if biased through the energy gap. At higher forward bias voltages, the tunnel diode performs like an ordinary junction diode. The tunnel diode is a low-noise, high speed device and has many applications, particularly in UHF technology.
As electronic devices, and storage devices in particular, have been formed at ever higher degrees of integration density, there has been an increased interest in the behavior of individual electrons and devices which are capable of exploiting this behavior. In many high capacity dynamic RAMs, for example, storage may be accomplished with only a few dozen electrons. Thus, individual electron behavior is of importance since leakage of only a single electron may represent a significant fraction of stored charge.
It has long been speculated and recently verified that, under the influence of particular electron transport mechanisms, electrons can move in a discrete fashion. Such a mechanism is provided when the electron passage is restricted to a small junction where the conduction mechanism is by quantum mechanical tunnelling. A theoretical discussion of single electron tunnelling is provided in CORRELATED DISCRETE TRANSFER OF SINGLE ELECTRONS IN ULTRA SMALL TUNNEL JUNCTIONS, by K. K. Likarev, IBM J. Res. Develop., Vol. 32, No. 1, January, 1988, pp. 144-158, which is hereby fully incorporated by reference. This paper particularly describes studies of conduction in metallic granular thin films at extremely low temperatures approaching that of liquid Helium. The observability of single electron tunnelling events is shown to be possible if the capacitance between metallic grains of the thin film is small, say 10.sup.-17 F or smaller and the energy, EQU E.sub.Q =e.sup.2 /2C,
associated with the transfer of charge from one metallic grain to another, which is not temperature dependent, becomes comparable to so-called shot noise arising from conduction in tunnel junctions at bias voltages exceeding a thermally dependent voltage EQU V=k.sub.B T/e.
Therefore, at any temperature such that EQU E.sub.Q &gt;k.sub.B T
can be satisfied, tunnelling becomes extremely unlikely at low bias voltages. As noted in the above-incorporated paper, at temperatures EQU T&lt;T.sub.0, where T.sub.0 .ident.e.sup.2 /2k.sub.B C,
no tunnelling happens if the difference in charge across the junction, Q, is an integral multiple of e, i.e. 0, 1, 2, 3, . . . This effect is known as the "Coulomb blockade of tunnelling" and the existence of this effect is crucial to observation of single electron tunnelling effects.
If, on the other hand, the charge difference across the capacitor is a half integral multiple of e, it is known that this Coulomb blockade disappears. The reason for this is that the charging energy of the system, Q.sup.2 /2C, is unchanged when the charge difference across the capacitor changes from -e/2 to +e/2, and thus there is no Coulomb barrier to overcome. The charge difference Q can be continuously changed by using a gate electrode or by change in the source-drain bias.
The possibility of observing and exploiting such single electron conduction effects has many implications, such as in the study or use of small but non-vanishing currents where discrete transfer of charge coexists with a quasi-continuous current which may be visualized by analogy to a dripping tap where the dripping consists of a plurality of discrete events but the flow in the conduit leading to the tap is a continuous, though small, flux. Other potentially useful possibilities include coherent tunnelling, sub-electron-charge control of DC current and digital electronics including single electron transistors and molecular circuit elements. Given the small geometric scale of such junctions, the potential for extremely large scale integration becomes clear.
The practical realization of even the possibility of study of these effects has been limited by the appearance that these effects occur in thin-film media only at liquid nitrogen temperatures and below, although some recent experiments involving two closely spaced but separate conductors have yielded results consistent with a Coulomb blockade effect at higher temperatures. However, at liquid nitrogen temperatures and below, single electron conductivity effects may be masked or otherwise disturbed by superconductivity effects. Further, any use or exploitation of these effects in a practical device would rely on the ability to reliably produce these effects at elevated (e.g. non-cryogenic) temperatures.